By default Active Directory requires authentication for all but the simplest searches. What's the recommended way to disable this policy? (I searched, but found only a few nonspecific Microsoft blog posts at first.)
The answer can be found in this article from Microsoft Technet. Search the page for 'Anonymous Queries'.
By default, anonymous LDAP operations to Active Directory, other than rootDSE searches and binds, are not permitted in Windows Server 2003. (Active Directory in Windows 2000 Server accepts anonymous requests; a successful result depends on objects having correct user permissions in Active Directory.)
To enable anonymous binding to Active Directory in Windows Server 2003, you must change the seventh character of the dsHeuristics attribute on the following directory object:
CN=Directory Service,CN=Windows NT,CN=Services,CN=Configuration,Root domain in forest
Valid values for the dsHeuristics attribute are 0 and 2. By default, the dsHeuristics attribute does not exist, but its internal default is 0. If you set the seventh character to 2, anonymous clients can perform any operation that is permitted by the access control list (ACL). If the attribute is already set, do not modify any bits in the dsHeuristics string other than the seventh bit. If the value is not set, make sure that you provide the leading zeros up to the seventh bit. You can use Adsiedit.msc to make the change to the dsHeuristics attribute.
After you set the dsHeuristics attribute, if you want anonymous users to be able to query Active Directory, you can enable anonymous access to specific directory objects. Users gain anonymous access to Active Directory objects through Anonymous Logon, which is a special security identifier (SID) that is used to represent anonymous network callers that perform an LDAP bind with NULL credentials.
Check out this article for Windows 2003 AD. Dunno about 2008, probably the same.